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Here the impulse arises from an ectopic focus, which may be anywhere in the auricles; it is not confined to these but spreads to the ventricles. The interval between the previous normal P wave and that of the premature beat is always less than the interval between normal beats and varies widely according to the degree of prematurity. The interval between the extrasystole and the next normal beat following varies also, and at no time does the sum of the two equal the duration of two normal beats. Therefore, the pause that follows an auricular extrasystole is not compensatory; it is too short; the T-P interval is shortened, and hence the fundamental rhythm is disturbed.The P wave is of abnormal contour, notched, splintered or inverted, owing to the abnormal path of the excitation wave in the auricles, which was generated in an ectopic (auricular) focus, at a
CARTER JB. THE FUNDAMENTALS OF ELECTROCARDIOGRAPHIC INTERPRETATION. JAMA. 1932;99(16):1345-1352. doi:10.1001/jama.1932.27410680006011